This invention relates to polyamide membranes prepared by interfacial polymerization of a water-soluble polyfunctional amine and a substantially water-insoluble polyfunctional acyl halide and methods for making the same.
Reverse osmosis membranes have been prepared from a variety of polymeric materials, including polyamides. Polyamide membranes are described in U.S. Pat. Nos. 3,567,632; 3,600,350; 3,687,842: 3,696,031: 3,878,109; 3,904,519; 3,948,823; 3,951,789: 3,993,625: 4,302,336; 4,337,154 and 4,387,024. These polyamide membranes are generally substantially linear polymers and exhibit relatively low water fluxes.
Asymmetric membranes are typically prepared by dissolving a polymer in a suitable solvent and casting the polymer in the form of films or fibers and then quenching the same in water to form the membranes. In addition, polyamide composite membranes have been described in the prior art. Examples of such membranes are disclosed in U.S. Pat. Nos. 3,744,642: 3,951,815; 4,005,012; and 4,039,440. Continuing developments in the art have led to improved membranes which can be used at relatively low transmembrane pressures. U.S. Pat. No. 4,259,183 describes certain composite membranes having a crosslinked polyamide discriminating layer. The discriminating layer is prepared by the interfacial polymerization of a water-soluble secondary amine and a water-insoluble polyfunctional acyl halide.
The preferred membranes desoribed in U.S. Pat. No. 4,259,183 possess certain unusual properties. These membranes exhibit a significant water flux at relatively low transmembrane pressures and also are ion selective. The patented membranes show much greater rejection of multivalent anion salts than of monovalent anion salts. Accordingly, such membranes are useful for applications such as water softening where it is desirable to remove such salts.
Other thin film composite polyamide membranes have been developed for specific reverse osmosis applications. U.S. Pat. No. 4,277,344 describes certain composite membranes having a discriminating layer prepared by interfacial polymerization of trimesoyl chloride optionally with other acyl halides and a monomeric aromatic diamine or polyamine. These membranes have been found to have superior water flux while maintaining a very high sodium chloride rejection in reverse osmosis applications.
Difficulties have been encountered in the preparation of composite polyamide membranes. Many substrate materials, for example polysulfone supports, must be properly wetted to promote uniform coating of the substrate with the aqueous phase involved in the interfacial polymerization. Surfactants, such as sodium lauryl sulfate, can be used to promote this wetting. However, the presence of surfactants can lead to undesirable discontinuities or defects in the discriminating layer. It is desirable to avoid too high a concentration of a conventional surface active agent, such as sodium lauryl sulfate, as this may deleteriously affect the desired interfacial polymerization. Other surfactants and wetting agents, for example polyvinyl alcohol, have been used in the manufacture of polyamide composite membranes. However, it has been found that these wetting agents may lead to reduced chemical resistance of the membrane discriminating layer.
A method of reproducibly fabricating membranes useful in water softening applications having a high water flux and good chemical resistance is desirable. In particular, membranes being tolerant of basic pH's, for example a pH of 12 or 13 for a period of several hours, are of commercial interest. Such basic solutions may be encountered in cleaning these membranes with conventional reagents.